): The overall objective of the proposed research is to delineate the mechanisms that regulate motility and invasive behavior of breast cancer cells. The proposal is based on a hypothesis that the components in the regulatory machinery of actin cytoskeleton organization play a critical role in breast cancer metastasis. These regulatory proteins may be used as the biomarkers for diagnosis and/or prognosis of metastatic breast cancer. Specifically, we postulate that the signaling pathway mediated by Abl interactor (Abi) proteins may play a key role in breast cancer metastasis, as this pathway is crucial for control of actin polymerization. To test this hypothesis, we will examine the expression, phosphorylation, complex formation, and subcellular localization of the proteins involved in Abi signaling in breast cancer cell lines as well as in paired specimens isolated from patients with breast cancer. A pathway-specific proteomic approach is designed to identify the components of Abi signaling that are abnormally expressed and/or modified in metastatic breast cancer cells. The role of Abi pathway in breast cancer metastasis will be tested by approaches using small hairpin RNA-mediated gene silencing and animal model. We anticipate that these approaches may yield critical data that will establish the clinical utility of the Abi pathway as a biomarker and target for metastatic breast cancer diagnosis and intervention. Understanding how the breast cancer cells escape the normal control of cell motility and become metastatic will help the development of new prognostic and therapeutic strategies for treatment of human breast cancer PUBLIC HEALTH RELEVANCE Breast cancer is the most common cancer in women in the United States and accounts for about 25% of all female malignancies worldwide. The lethality of breast cancer is largely due to the metastasis of the tumor cells to distant sites including lymph nodes, bone, liver, brain, and lung. A major challenge in clinical management of breast cancer has been to improve the ability to detect metastatic development at the early stage. The proposed research is aimed at the improving our understanding of the molecular mechanisms of the metastatic process of breast cancer and developing novel prognostic markers for early detection of metastatic breast cancer. The research focuses on a complex of molecules critical for regulation of fundamental cellular process required for cell migration and invasion. The state-of-art techniques will be employed to identify novel diagnosis and prognosis markers. These novel biomarkers will be tested in a cohort of breast cancer patients to determine their clinical utility. Furthermore, the role of these molecules in development of breast cancer metastasis will be tested in an animal model. Understanding how the breast cancer cells escape the normal control of cell motility and become metastatic will help the development of new prognostic and therapeutic strategies for treatment of human breast cancer